1. Field of the Invention
This invention relates to an optical type displacement sensor for finding a physical amount such as the amount of movement or displacement of an object highly accurately by the utilization of the fact that interfering light beams are modulated by diffraction and interference occurring when light is applied to the object.
2. Related Background Art
Optical type displacement sensors of this type according to the prior art include optical encoders, laser Doppler speed meters, laser interferometers, etc. and these are highly accurate and have high resolving power, but in order that they may be applied to a wider field, compactness (a size of the order of millimeters), higher accuracy, higher resolving power (of the order of 0.1 .mu.m) and high stability are required of them. A sensor of a size of millimeters could be directly attached to an object to be measured for use and therefore could also be used in a compact apparatus, but is liable to cause an attaching error and thus, it becomes necessary to cope with it.
As the prior art effective for miniaturizing in the field of such a detecting apparatus utilizing light, there are the following.
FIGS. 1A and 1B of the accompanying drawings are illustrations of an optical encoder disclosed in Japanese Laid-Open Utility Model Application No. 1-180615. A light beam diverging from a light emitting element 42 passes through an aperture 46A in a base plate 46, is converted into a linear ray array by a slit array 14 and is applied to a grating on a scale 40, whereupon the grating of the scale 40 is projected onto an index grating 16 by a light beam reflected from the bottom 12 of the scale 40, and the quantity of light entering a light receiving element 48 on the base plate 46 is modulated by the geometrical superposition of the two. This is the principle of this optical encoder. According to this principle, downsizing is possible, but resolving power is limited.
FIGS. 2A and 2B of the accompanying drawings are illustrations of an optical type encoder disclosed in Japanese Laid-Open Patent Application No. 62-121314, and this encoder is an effective example of an improved construction for downsizing the basic optical system (British Laid-Open Patent No. 1,474,049) of an encoder using three diffraction gratings. A light beam diverging from a light emitting element 51 is made into a parallel light beam by a lens 52, and is diffracted when it is applied to a grating GK(A) on an index scale A, and creates light beams in three directions of emergence.
Each light beam is diffracted by a grating GK(B) on a scale B, is subjected to phase modulation by relative movement and is returned to the grating GK(A) on the index scale A, and three sets of interfering light beams are caused to enter light receiving elements provided in different azimuths, by the diffraction by the index grating. By this construction, compactness and high resolving power are made compatible.
FIG. 3 of the accompanying drawings are illustrations of an optical type encoder disclosed in Japanese Laid-Open Patent Application No. 3-279812, and this encoder is an example which is effective to achieve high accuracy, simplicity and compactness at a time. In FIG. 3, the reference numeral 61 designates a light emitting element, the reference numeral 62 denotes a lens, the reference characters 63, 63.sub.1a, 63.sub.1b and 64 designate diffraction gratings, and reference characters 65a and 65b denote light receiving elements. The letter L designates an incident ray of light, and rays of light denoted by L( ) are diffracted lights. The reference characters 1a and 1b denote the interfering light beams of diffracted lights finally emerging from the diffraction gratings 63.sub.1a and 63.sub.1b. These interfering light beams are detected by the light receiving elements 65a and 65b, respectively, whereby the amount of relative displacement between the diffraction gratings can be detected.
However, the encoders according to the prior art as optical type displacement sensors are all made compact and highly accurate, but they suffer from the following problems.
(1) The light emitted from the light emitting element is horizontally divided into two or more and the interfering light beams thereof are received by the photoelectric element. Thus, it is very difficult to adjust all of the interfering states of the interfering light beams combined by the diffraction gratings into equal constant states. Therefore, the amplitude and phase difference of the signal obtained between different interference areas are not stable.
(2) The light beams divided in one and the same interfering light beam become more liable to fluctuate in their interfering state due to the influence of an attaching error as the resolving power of the main body is more improved, and the phase difference outputted from each phase becomes unstable. As long as such a construction is adopted, it is practically difficult to construct a displacement sensor of high resolving power.